The use of computers to control hydraulic systems in the suspension of motor vehicles has been discussed and various arrangements for controlling the suspension and for construction of the hydraulic actuator have been described and patented.
U.S. Pat. No. 4,333,668 issued Jun 8, 1982 to Hendrickson et al discloses a shock absorber with damping orifices that are controlled by solenoids which in turn are activated by a computer control in response to the rate of change of the extension of the absorber. Pitch and roll of the vehicle are also imposed on the control which energizes the solenoid to vary the opening and closing of the valves and maintain the vehicle substantially stable.
U.S. Pat. No. 4,639,013 issued Jan. 27, 1987 to Williams et al describes an active vehicle suspension incorporating a double acting hydraulic actuator in parallel with a gas spring. An actual change in load is sensed and an appropriate adjustment of the actuator is made to compensate for the sensed load change.
Canadian patent 1,230,657 issued Dec. 22, 1987 to Williams et al describes an active vehicle suspension system using hydraulic actuators for each wheel to generate signals in accordance with their displacements and forces applied thereto and controls the displacement of the hydraulic actuator in accordance with the interpretation of these signals to maintain vehicle stability.
U.S. Pat. No. 4,753,328 issued Jun. 28, 1988 to Williams is a further modification of the systems described in the preceding William's U.S. and Canadian patents and further discloses a damping system to selectively apply positive or negative damping to the movement of the pistons of the actuators.
Digital hydraulic actuators are also known. U.S. Pat. No. 4,602,481 issued Jul. 29, 1986 to Robinson describes a particular form of digital actuator utilizing piston areas of different sizes to selectively apply forces of a preselected magnitude. The force applied is controlled by adjusting the ratio of piston area subjected to the source pressure to that subjected to return pressure. The pressures may selectively be applied to force the actuator in opposite directions. By changing the piston area subjected to source (higher) pressure driving the actuator in one direction relative to the area under similar pressure driving the actuator in the opposite direction one can adjust the force to be positive in either direction and to have a selected value depending on the combination of areas subjected to source pressure or return pressure.